This invention relates to an improved implosion protection system for a color-cathode ray tube bulb. A conventional color cathode ray tube bulb includes a faceplate with a rearward flange and a funnel sealed to the faceplate flange along a planar sealing interface. Due to the high vacuum in the bulb, several tons of atmospheric pressure are exerted on the bulb, causing it to be susceptible to implosion. (The term "implosion" is defined by Underwriters Laboratory Inc. as a "rapid and sudden inward bursting of a high-vacuum glass envelope.") It is of utmost importance in the interest of safety to prevent the bulb from violently imploding should it, for example, be struck by a heavy missile. There have evolved a number of approaches to implosion protecting color cathode ray tube bulbs of the type described having a flanged faceplate. A first approach seeks to confine and restrain the shards of a fractured faceplate -- typically by the use of a circumscribing frame which is cemented to the faceplate flange with an epoxy cement. This is the so-called "rimbond" type implosion protection system.
A second approach involves the use of a split frame which fits over the faceplate flange and which is constricted tightly against the flange by means of a tension band. The frame may be "wet", that is, cemented to the faceplate flange, or "dry".
A third approach involves laminating a transparent protective shield over the viewing window of the faceplate.
A fourth approach, and the one with which this invention is concerned, is the so-called "tension band" approach. Implosion protection is provided in conventional tension band systems by one or more high tension bands which are placed around the faceplate flange and drawn up to a very high tension, for example, 1500-2000 p.s.i. The principle of operation is not primarily one of confinement, as in rimbond systems, but one of pre-load. By compressively pre-loading the faceplate flange, the implosion-induced tensive stresses in the faceplate must first overcome the compressive pre-stresses in the faceplate before the faceplate will fracture.
Tension bands implosion systems are popular commercially, in applications where they are efficacious, because of their low cost. They do, however, have a number of serious shortcomings. They suffer from their provision of only marginal implosion protection in certain applications; in other applications they are completely impotent. Specifically, it has been found that tension band systems are completely inadequate in providing implosion protection on bulbs of certain sizes and configurations. Examples of prior art United States patents issued on various aspects of tension band systems are Nos. 3,818,557; 3,456,076; 3,556,306; 3,597,537; 3,777,057; 3,845,530; and 3,890,464.
U.S. Pat. No. 3,605,227-Nienhuis et al depicts an implosion protection system of the tension band type in which an "annular strap" (band) of the same size or slightly larger than the circumference of the faceplate flange is placed about the faceplate flange (Col. 1, line 60). Tension is introduced in the band, allegedly without the disadvantages of non-uniform tensile force distribution found in prior art systems, by driving cushioned "wedge-shaped parts" between the strap and the outer surface of the faceplate flange (Col. 1, line 65). The wedge-shaped parts and the strap are preferably then affixed to the glass as by the use of a cement such as a hardening synthetic resin which is cast in the spaces between the wedge-shaped parts and the glass (Col. 1, line 72). This prevents the dislodging of the wedges by shocks and the like, it is said.
A wedge-shaped part is illustrated at a corner of the faceplate (Nienhuis FIG. 2), but it is said that the parts are "preferably arranged at the center of the straight sides or are distributed uniformly over the straight sides, since the force required for inserting the wedges to obtain a given tensile force in the strap may be comparatively small. Moreover, an additional pressure is then exerted on the glass along the straight sides, which has a favorable influence on the reinforcement of the glass surface of these flat sides located between mold match line and the front of the window" (Col. 2, line 15).
Placement of the wedge-shaped part at the center of the straight sides has a third advantage, it is said: the strap is spaced from the glass by a larger distance which facilitates the casting of adhesive and filler materials (Col. 2, line 27).